Of the various types of fire suppression systems, the dry pre-action system finds widespread use, especially in facilities where it is important to avoid accidental or inadvertent activation. Typical applications for dry pre-action systems include museums, libraries and computer centers, where water damage to property is a serious consideration. Such systems are also suitable for residential use as applied by NFPA 13, 13R and 13D including applications to concealed space as well as attic applications.
Dry pre-action systems comprise a piping network that extends throughout the building or other structure to be protected. The network is in fluid communication with a source of pressurized fire suppressant, typically water from a service main. Sprinklers in fluid communication with the piping network are distributed along the network. The sprinklers are normally closed, but open to discharge the water in response to heat from a fire, often through the use of a fusible link, such as a heat sensitive glass bulb or a mechanism held together by a solder having a predetermined melting point.
The system is known as “dry” because water is not normally present in the piping network. Water flow to the network is controlled by a control valve which is opened in response to a fire condition. There are two dominant methods used to open the control valve, the single interlock and double interlock systems. In the single interlock system, a single event, such as the activation of a fire detection sensor (for example, a smoke detector, heat detector, flame detector, temperature sensor or other type of sensors) or the opening of a sprinkler, triggers the opening of the control valve providing water to the system. In the double interlock system, two events indicative of a fire, such as the activation of a fire detection sensor and the opening of a sprinkler must occur contemporaneously to trigger opening of the control valve.
Dry pre-action fire suppression systems, both single interlock and double interlock type, often rely on AC power for operation of various electrical and electronic components comprising the system. For example, the system may have a microprocessor based electronic control system, relays, solenoid valves and electrically powered sensors. If AC power is lost then the system is non-functional and there is no fire protection. To avoid this situation battery back-up power is provided. This is effective as long as the battery power is available. If the AC power outage outlasts the battery life however, the problem of a non-functioning fire suppression system, and an absence of fire protection, remains a serious concern and is unacceptable in many situations.
There is clearly a need for a fire suppression sprinkler system which can be automatically converted, in the event of a power failure, from one which depends on electrical power, to one which is independent of electrical power, either AC or battery back-up.